Memory devices are typically provided as internal, semiconductor, integrated circuits in a wide range of processing applications, such as computers or other electronic devices. There are many different types of memory devices including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), flash memory, “phase-change” random access memory (PCRAM), and resistance random access memory (RRAM), among others. Memory devices can be comprised of an array arrangement of memory cells.
A memory cell stores digital information in a structure that can be rapidly switched between more than one readily discernable state. Some memory cells are based on the presence or absence of electrical charge contained in a region of the cell. By retaining its charge, the memory cell retains its stored data. Some memory cell structures inherently leak charge, and must be continually powered to refresh the stored charge.
Non-volatile memory however, does not require electrical power to retain charge information. For example, flash memory typically has a “floating gate” upon which the charge is stored, which is insulated to minimize charge leakage. Thus, power is required only to change the stored information, e.g., write-to (store charge), read-from (determine if charge is present), or erase (remove charge) data. The non-volatility of stored data in flash memory is advantageous in portable electronic applications. Uses for non-volatile memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data, such as a basic input/output system (BIOS) used in personal computer systems (among others), are typically stored in non-volatile memory devices.
“Phase change” memory cells use detectable changes in physical structure of the memory cell material to define various states, e.g., resistance changes associated with different molecular structures of the memory cell material. The various states can be associated with digital information. The physical layout of a PCRAM or RRAM memory cell within a memory device array may be arranged similarly to a DRAM memory cell; however, the capacitor of the DRAM cell is replaced by a material having detectable “phase change” characteristics, e.g., resistance states.
Memory cells in an array architecture can be programmed to a desired resistance state. A single “phase change” memory cell may have more than two discernable “phase” states, each “phase” state having a corresponding different resistance state, and thus may store more than two data values, e.g., digits. Such memory cells may be referred to as multi state memory cells, multidigit cells, or multilevel cells (MLCs). MLCs can allow the manufacture of higher density memories without increasing the number of memory cells since each cell can represent more than one binary digit, e.g., more than one bit. MLCs having more than one programmed state, e.g., a memory cell capable of representing two digits can have four programmed states, a cell capable of representing three digits can have eight program states, etc.